1. Field of the Invention
The present invention relates generally to medical stimulators and more specifically to implantable cardioverters and defibrillators.
2. Description of the Prior Art
When implanting an implantable defibrillator, it is desirable to test the device's operability to ensure that it is capable of reliably defibrillating the heart. In order to accomplish this, it is necessary to first induce fibrillation in the patient's heart, and then determine whether the implantable defibrillator is capable of terminating the induced fibrillation. Typically, rapid pacing from the implanted defibrillator or an external pulse generator or an external 60 Hz low voltage transformer has been used in inducing fibrillation.
It would be desirable to improve the fibrillation induction function in an implantable defibrillator, to allow for a more fully automated testing regimen and to simplify the implantation procedure. However, incorporation of a true 60 Hz alternating current fibrillator into an implantable device poses substantial technical difficulties.
U.S. Pat. No. 5,129,392 to Bardy et al discloses an automatic fibrillator for inclusion in an implantable defibrillator. Fibrillation is induced using overdrive pacing. The effective refractory period of the patient's heart is measured during pacing. A fibrillation inducing pulse is delivered at a calculated time interval following an overdrive pacing pulse. This approach requires complex tinting calculations and may not be effective in all cases in inducing fibrillation.
U.S. Pat. No. 5,215,083 to Drane et al discloses an apparatus and method for fibrillation induction. At least a portion of a pulse train of "micro-shocks" delivered from the high voltage capacitors are synchronized to the patient's T-waves. The length of the series of trains and the micro-shock pulse widths are programmable parameters and the polarity of the shocks may be alternated within a train. This technique is also complex and may not be consistently effective.
Copending U.S. patent application Ser. No. 08/568,044, filed Dec. 6, 1995, to Fain et al. and assigned to the assignee of the present application discloses a method and apparatus for inducing fibrillation in a patient's heart by delivering a direct current shock to the heart from a DC-to-DC converter. The hardware of a conventional implantable cardioverter/defibrillator (ICD) is utilized with a modification to the control algorithms. The DC-to-DC converter which is normally used to charge the ICD high voltage capacitors is activated and immediately thereafter or following a short period to allow the high voltage capacitors to charge, the high voltage output switches of the output stage are closed. This delivers the output current from the DC-to-DC converter to the defibrillation electrodes and through the patient's heart. This pulse or shock is continued for a predetermined time of between about 30 milliseconds to 5 seconds. At that point the output switches are opened and the converter is shut off. This DC stimulus delivered directly to the patient's heart induces fibrillation. A problem with this system is that the voltage available from the output of the DC-to-DC converter may not be sufficiently high to consistently induce fibrillation.
It is therefore an object of the invention to provide an improved method and apparatus for inducing fibrillation in a patient's heart.